Various DC-DC converters having a half-bridge node, arranged between a low-side switch (LSS) and a high-side switch (HSS) are known. Such converters are used, for example for powering compact fluorescent lighting, or adaptors for PC and TV applications and can be provided in various configurations, such as resonant converters, for example so-called LLC, or LCC converters. FIG. 1 shows a block diagram of an LLC converter. The controller 100 includes a high side part 110 and a low side part 120.
The controllers for such converters must be able to control the high-side switch and the low-side switch. Typically, to be able to control the HSS (shown in FIG. 1 as T1), even when the LSS is open, the high side part of the controller needs its own individual supply, which is normally realized from a capacitor, shown at C1 charged via a so-called bootstrap configuration: the capacitor is charged during the time the LSS (T2) of the converter is in ON state (that is to say, T2 is closed). During that time, the voltage at the half—bridge node (V_hb) is pulled low, so the capacitor can be charged from the supply voltage (V_suphs), which is supplied from the DC-power supply 130 through a diode D1.
The moment the LSS is switched OFF (that is to say, opened), these applications will intentionally lift the so called ‘floating ground’ level of the high side part 110 due to the inductive current (in the inductor L1), leaving its supply solely to the charged bootstrap capacitor. Now, the bootstrap capacitor C1 is discharged by the high side part drawing power to operate its various functions. Of course, the most fundamental of these is to open and close the HSS T1—which are transitory actions—but may include other functions. At the next cycle, the LSS is turned on again, connecting the half-bridge node to ground, and the bootstrap capacitor is recharged.
In some modes of operation, it may occur that the particular steady state where both the HSS and LSS are OFF for a longer period and the voltage at the half bridge node V_hb is floating, the bootstrap capacitor can be fully discharged by the high side part. Burst mode operation is such a mode, in which the HSS and LSS may both be OFF for a relatively long period, for example tens of milliseconds, typically up to around 100 ms. As a consequence, the system has to start by turning on the low side part to assure recharge of the bootstrap capacitor, before the high side part can provide proper control of the high-side switch. This may be undesirable.